The prior art in current use in this area is either the slurry feed method or the batch process feed method (commonly known as the lockhopper method). In the slurry feed method, a liquid-solid mixture is pumped into the pressure vessel using more or less conventional pumps. With this arrangement the liquid (usually water) required to transport the solid material may not be required in the process and therefore will reduce process efficiency or require considerable effort to be removed before processing can begin. The previous dry feeding arrangement, the batch process feed method, is to load the material into a hopper, close and pressurize the hopper with gas, and then dump the material into the pressure vessel. The hopper remains filled with high pressure gas which must be vented to prepare the hopper for loading the next batch of material. Such venting and batch feeding of material is not desirable. Large valves at the inlet and outlet of the hopper are required; such valves, operating cyclically in the environment of abrasive pulverized solids, have a poor effective life and reliability. Moreover, the pressurization gas contains suspended particles of the material and cannot be vented until it is processed to remove the suspended material.
Other prior art centrifugal device designs have been proposed for delivering dry pulverized coal to a high pressure chamber. Such prior art has many serious drawbacks. First, many of these prior art centrifugal devices (Staudinger U.S. Pat. No. 4,049,133, Duch U.S. Pat. No. 4,034,870, van der Burgt U.S. Pat. No. 4,120,410) reflected concern that the coal would form bridges and plug the rotor channels if it was allowed to compact. Therefore, there was an attempt to arrange for the material to remain in a fluidized state all the way through the rotor, as opposed to the compacted plug type flow in the present invention. Fluidized means that the material particles are separated by gas pressure forces so that there is little frictional stress between solid particles. Unfortunately, the fluidized flow of solids cannot be stabilized when pumping against a significant pressure. Furthermore, this attempt to avoid compaction and plugging was misguided. As is well known in the field of gravity flow of bulk (non-fluidized) solids, stable bridges or domes will not form if the dome span required is larger than a critical diameter, which is dependent on the bulk material properties. If the channel is smaller than the critical diameter, plugging can be expected. A typical lg critical diameter for finely ground coal would be perhaps 10-20 cm. However, the key unrecognized fact is that the critical diameter is inversely proportional to the body force (i.e., g-force) acting on the material and consequently there is no serious plugging problem in even very narrow rotor channels. This can be shown from the mechanics of dome formation and is verified by experience. Thus, common experience with bin or hopper plugging problems with cohesive materials, which is obtained under 1 g gravity, is not a valid guide to flow or non-flow under several thousand g's which is typical of the present invention.
Also, none of the prior centrifugal device patents known to applicants have any structure which adequately stabilizes the flow of solids. Nor do they attempt any separation of the control of the flowrate, or metering, from the pressure sealing function. Most used some sort of spring loaded valve or flexible structure to attempt to control the efflux of coal from the rotor (Staudinger U.S. Pat. No. 4,049,133; Duch U.S. Pat. No. 4,034,870; Zellerhoff U.S. Pat. No. 3,182,825). Also, Duch U.S. Pat. No. 4,034,870 and van der Burgt U.S. Pat. No. 4,120,410 propose a cupped cymbal rotor structure where the efflux gap is a continuous slit around the rim and is adjustable by moving the two cups relative to one another. None of these prior structures successfully decouple the coal metering from the pressure sealing functions, which is of critical importance.
Applicants' preliminary analysis and experimental work preceding the development of the present invention is set forth in the Proceedings of the Conference on Coal Feeder Systems, published by the Jet Propulsion Laboratory, June 21 through 23, 1977, and the Coal Feeder Development Program, Phase II Report, FE-1792-34, dated July 1977 and approved for publication Oct. 31, 1977, Department of Energy, United States Government.